When cell cultures are scaled up from a small cell bank sample to larger production batches, this normally has to be made in several steps, using separate bioreactors. This sequence of cultivations is often called a seed train and is needed in order to keep the cell densities within a certain optimal window, often in the order of>105 cells per ml. In large scale production of biopharmaceuticals where the scale-up is from a small cryopreserved vial sample up to several m3, the seed train can involve up to six steps and take several weeks. It is also a complex procedure in that sterile transfer of the culture from one bioreactor to another is needed, such that transfers have to be made in LAF benches or sterile cleanrooms. Even under these conditions there is also a certain risk for adventitious infections, which can have disastrous consequences for a high value large scale culture. Similar concerns apply also for expansion of cells, e.g. stem cells, to be used in clinical cell therapy.
With the general trend towards single-use vessels for cell culture, there is an increasing tendency towards using flexible bag bioreactors in seed trains. However, the need to empty one smaller bag and to transfer the content into a larger bag still remains and is a work-intensive operation with some risk of contamination. It has been suggested to gradually increase the culture volume in flexible bags either by clamping off part of the bag over the entire bag cross section and then removing the clamp (WO2008153401) or by starting with a folded bag and then unfolding it ((US20100055764). However, these solutions do not provide good sealing between the used and unused compartments, leading to leakage of culture into the unused compartments and contamination of the cell culture by substances released from cells grown under unsuitable conditions. These methods also involve a substantial risk of mechanical damage to the bags, with resulting risks of bag rupture.
Accordingly, there is a need for a safe and convenient way of transferring cell cultures from one flexible culture compartment to another under sterile conditions. There is also a need for accurate temperature control of sensitive cell cultures, particularly in small volumes.